Metal halide perovskite photovoltaic cells could potentially boost the efficiency of commercial silicon photovoltaic modules from ~ 20 toward 30% when used in tandem architectures. An optimum perovskite cell optical band gap of ~1.75 electron volts (eV), can be achieved by varying halide composition but to date, such materials have had poor photostability and thermal stability. Here, we present a highly crystalline and compositionally photostable material, [HC(NH 2 One concept for improving the efficiency of photovoltaics (PVs) is to create a "tandem junction," for example, by placing a wide band gap "top cell" above a silicon "bottom cell." This approach could realistically increase the efficiency of the Si cell from 25.6% to beyond 30% (1, 2). Given the crystalline silicon (c-Si) band gap of 1.1 eV, the top cell material requires a band gap of ~1.75 eV, in order to current-match both junctions (3). However, suitable wide-band-gap top-cell materials for Si or thin film technologies that offer stability, high performance, and low cost have been lacking. In recent years, metal halide perovskite-based PVs have gained attention because of their high power conversion efficiencies (PCE) and low processing cost (4-11). An attractive feature of this material is the ability to tune its band gap from 1.48 to 2.3 eV (12, 13), implying that we could potentially fabricate an ideal material for tandem cell applications.Perovskite-based PVs are generally fabricated with organic-inorganic trihalide perovskites with the formulation ABX 3 , where A is the methylammonium (CH 3 NH 3 ) (MA) or formamidinium (HC(NH 2 ) 2 ) (FA) cation, B is commonly lead (Pb), while X is a halide (Cl, Br, and I). Although these perovskite structures offer high power conversion efficiencies (PCE), reaching > 20% PCE with band gaps of around 1.5 eV (14), fundamental issues have been discovered when attempting to tune their band gaps to the optimum 1.7 to 1.8 eV range. In the case of MAPb(I(1-x)Brx) 3 , Hoke et al. reported that light-soaking induces a halide segregation within the perovskite (15), The formation of iodiderich domains with lower band gap result in an increase in sub-gap absorption and a red-shift of photoluminescence (PL). The lower band gap regions limit the voltage attainable with such a material, so this band gap "photoinstability" limits the use of MAPb(I(1-x)Brx) 3 in tandem devices (15). In addition, when considering real-world applications, MAPbI3 is inherently thermally unstable at 85°C, even in an inert atmosphere (international regulations require a commercial PV product to withstand this temperature) (16).

Three-dimensional (3D) organic-inorganic perovskite solar cells have undergone a meteoric rise in cell efficiency to > 22%. However, the perovskite absorber layer is prone to degradation in water, oxygen and UV light. Two-dimensional (2D) Ruddlesden−Popper layered perovskites have exhibited promising environmental stability, but perform less well in solar cells, possibly due to the inhibition of out-of-plane charge transport by the insulating spacer cations. Alternatively, moving away from methylammonium, to the mixed cation formamidinium-caesium based perovskites has led to considerably enhancement of the stability of 3D perovskite absorber layers. Here, we report highly efficient and stable perovskite solar cells based on a self-assembled butylammonium-Cs-formamidinium mixed-cation lead mixed-halide perovskite photoactive layer. Long-chain alkyl-ammonium halides added to the formamidinium-cesium based perovskite precursor solution strongly enhances the crystallinity of the 3D perovskite phase, while also inducing the formation of new layered-phases in the films. By carefully regulating the composition, we are able to achieve "plate-like" layered perovskite crystallites standing up between the host 3D perovskite grains. This spontaneously forming heterostructure allows the efficient charge carrier transport in the 3D perovskite phase, while reducing charge recombination via fortuitous grain boundary passivation. We also observe reduced current-voltage hysteresis and improved device stability, which we correlate to enhanced crystallinity and reduced crystal defects in the 3D perovskite phase. With the optimized composition, we achieved a power conversion efficiency of 20.6% (stabilised efficiency of 19.5%) from a narrow bandgap (1.61 eV) perovskite solar cell and of 17.2 % (stabilised efficiency of 17.3%) from a wider bandgap (1.72 eV) perovskite solar cell optimised for tandem applications. In addition to enhanced efficiency, the addition of butylammonium greatly enhances the long-term stability of the devices. For the first time, our cells sustain more than 80% of their "post burn-in" efficiency after 1,000 hrs of aging under simulated full spectrum sun light measured in an ambient environment without encapsulation. With additional sealing with a glass/polymer-foil/glass laminate, we extend this lifetime to close to 4,000 hrs. Our work illustrates that engineering heterostructures between 2D and 3D perovskite phases is both possible, and can lead to enhancement of both performance and stability of perovskite solar cells.

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Lead-based halide perovskites are emerging as the most promising class of materials for next generation optoelectronics. However, despite the enormous success of lead-halide perovskite solar cells, the issues of stability and toxicity are yet to be resolved. Here we report on the computational design and the experimental synthesis of a new family of Pb-free inorganic halide double-perovskites based on bismuth or antimony and noble metals. Using first-principles calculations we show that this hitherto unknown family of perovskites exhibits very promising optoelectronic properties, such as tunable band gaps in the visible range and low carrier effective masses. Furthermore, we successfully synthesize the double perovskite Cs 2 BiAgCl 6 , we perform structural refinement using single-crystal X-ray diffraction, and we characterize its optical properties via optical absorption and photoluminescence measurements.This new perovskite belongs to the Fm3m space group, and consists of BiCl 6 and AgCl 6 octahedra alternating in a rock-salt face-centered cubic structure. From UV-Vis and PL measurements we obtain an indirect gap of 2.2 eV. The new compound is very stable under ambient conditions. Table of Contents ImageKeywords: Noble-metal halide double perovskites, Lead-free perovskites, Computational design, materials synthesis, structure refinement, UV-Vis spectra, Photoluminescence spectra 2 Perovskites are among the most fascinating crystals, and play important roles in a variety of applications, including ferroelectricity, piezzoelectricity, high-T c superconductivity, ferromagnetism, giant magnetoresistance, photocatalysis and photovoltaics. [1][2][3][4][5][6][7][8] The majority of perovskites are oxides and are very stable under ambient temperature and pressure conditions. 4,9 However, this stability is usually accompanied by very large band gaps, therefore most oxide perovskites are not suitable candidates for optoelectronic applications. The most noteworthy exceptions are the ferroelectric perovskite oxides related to LiNbO 3 , BaTiO 3 , Pb(Zr, Ti)O 3 and BiFeO 3 , which are being actively investigated for photovoltaic applications, reaching power conversion efficiencies of up to 8%. 9 The past five years witnessed a revolution in optoelectronic research with the discovery of the organic-inorganic lead-halide perovskite family. These solution-processable perovskites are fast becoming the most promising materials for the next generation of solar cells, achieving efficiencies above 20%. [10][11][12][13] Despite this breakthrough, hybrid lead-halide perovskites are known to degrade due to moisture and heat, 14 upon prolonged exposure to light, 15 and are prone to ion or halide vacancy migration, leading to unstable operation of photovoltaic devices. 16,17 At the same time the presence of lead raises concerns about the potential environmental impact of these materials. 18,19 Given these limitations, identifying a stable, non-toxic halide perovskite optoelectronic material is one of the key challenges to be ad...

Room-temperature films of black-phase caesium lead iodide (CsPbI3) are widely thought to be trapped in a cubic perovskite polymorph. Here, we challenge this assumption. We present structural refinement of room temperature black-phase CsPbI3 in an orthorhombic polymorph. We demonstrate that this polymorph is adopted by both powders and thin-films of black-phase CsPbI3, fabricated either by high-or low-temperature processes. We perform electronic band structure calculations for the orthorhombic polymorph and find agreement with experimental data and close similarities with orthorhombic methylammonium lead iodide. We investigate the structural transitions and thermodynamic stability of the various polymorphs of CsPbI3, and show that the orthorhombic polymorph is the most